Systems and methods for transmural ablation

ABSTRACT

A method of applying ablation energy to achieve transmurality including applying ablation energy at a starting power to a tissue site and monitoring the impedance of the tissue site. A power applied to the tissue site can be reduced as a function of a rate of an increase in impedance according to some embodiments.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 11/780,911, filedJul. 20, 2007, that claims priority to U.S. Provisional Ser. No.60/832,242 filed Jul. 20, 2006, and U.S. Provisional Ser. No. 60/923,365filed Apr. 13, 2007, and which is continuation-in-part of U.S. Ser. No.10/923,178, filed Aug. 20, 2004, now U.S. Pat. No. 7,250,048, which is acontinuation-in-part of U.S. Ser. No. 10/364,553, filed Feb. 11, 2003,now U.S. Pat. No. 6,989,010, which is a continuation-in-part applicationof U.S. Ser. No. 10/132,379, filed Apr. 24, 2002, now U.S. Pat. No.6,648,883, which claims priority to U.S. Provisional Ser. No.60/287,202, filed Apr. 26, 2001, the contents of each of which arehereby incorporated by reference in their respective entireties.

FIELD

Embodiments of the invention related generally to systems and methodsfor ablating tissue, and more particularly, to systems and methods forperforming transmural ablations.

BACKGROUND

The Maze procedure is a surgical treatment for patients with chronicatrial fibrillation that is resistant to other treatments. The Mazeprocedure uses incisions in the right and left atria to divide the atriainto electrically isolated portions, which in turn results in an orderlypassage of the depolarization wave front from the sino-atrial node tothe atrial-ventricular node, while preventing reentrant wave frontpropagation. Although successful in treating atrial fibrillation, theMaze procedure can be quite complex and is currently performed by alimited number of highly skilled cardiac surgeons in conjunction withother open-heart procedures. As a result of the complexities of the Mazeprocedure, there has been an increased level of interest in proceduresemploying electrosurgical devices or other types of ablation devices,(e.g., thermal ablation, micro-wave ablation, radio frequency or RFablation, and cryo-ablation) to ablate tissue along pathwaysapproximating the incisions of the Maze procedure.

Three basic approaches have been used to create elongated lesions withelectrosurgical devices. The first approach is to create a series ofshort lesions using a contact electrode, moving it along the surface ofthe organ wall to be ablated to create a linear lesion. This can beaccomplished either by making a series of lesions, moving the electrodebetween lesions, or by dragging the electrode along the surface of theorgan to be ablated and continuously applying ablation energy. Thesecond approach to creation of elongated lesions is to use an elongatedelectrode and to place the elongated electrode along the desired line oflesion along the tissue. The third approach to creation of elongatedlesions is to provide a series of electrodes and arrange the series ofelectrodes along the desired line of lesion. The electrodes may beactivated individually or in sequence. In the case of multi-electrodedevices, individual feedback regulation of ablated energy applied viathe electrodes may also be employed.

In conjunction with the use of electrosurgical ablation devices, variouscontrol mechanisms have been developed to control the delivery ofablation energy to achieve the desired result of ablation (i.e., killingof cells at the ablation site, while leaving the basic structure of theorgan to be ablated intact). Such control systems include measurement oftemperature and impedance at or adjacent to the ablation site, asdisclosed in U.S. Pat. No. 5,540,681, issued to Struhl, et al.

Additionally, there has been substantial work done toward assuring thatthe ablation procedure is complete, i.e., that the ablation extendsthrough the thickness of the tissue to be ablated, before terminatingapplication of ablation energy. This desired result is sometimesreferred to as a “transmural” ablation. For example, detection of adesired drop in electrical impedance of the tissue being ablated at theelectrode site as an indicator of transmurality is disclosed in U.S.Pat. No. 5,562,721 issued to Marchlinski et al. Alternatively, detectionof an impedance rise or an impedance rise following an impedance fall isdisclosed in U.S. Pat. No. 5,558,671 issued to Yates and U.S. Pat. No.5,540,684 issued to Hassler, respectively.

Previous transmurality algorithms were fundamentally based on theconcept of identifying a flat impedance curve or plateau in response toan increase in power of ablation energy output to determinetransmurality. However, there are many situations in which the flattenedimpedance curve does not remain plateaued long enough for the algorithmto determine that the flattened impedance curve indicates transmurality.The ablation is allowed to continue, which can sometimes cause theimpedance curve to rise as a result of increased temperature (thisusually occurs in fatty, inhomogeneous, or thicker tissues). Therefore,ablation is not terminated until the detection of an impedance risefollowed by a minimum time delay or by reaching the high impedancelimit. This is an inefficient method for performing a transmuralablation and can result in over-ablation.

SUMMARY

In one embodiment, the invention provides a method of applying ablationenergy to achieve transmurality at a tissue site including applyingablation energy at a first power to the tissue site, monitoringimpedance of the tissue site, and reducing the ablation energy appliedto the tissue site to a second power as a function of a rate of increaseof an identified rising impedance.

Other aspects of the invention will become apparent by consideration ofthe detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart depicting an algorithm for determiningtransmurality according to one embodiment of the invention.

FIG. 2 is a flowchart depicting a power modulation algorithm accordingto one embodiment of the invention.

FIG. 3 is a chart illustrating an impedance versus power curve.

FIG. 4 includes two charts and two schematic diagrams illustrating animpedance profile and power output for positive closed loop feedback (a)and positive/negative closed loop feedback (b) systems.

FIG. 5 is a chart illustrating a comparison of time to first powerplateau for various starting powers.

FIG. 6 is a chart illustrating a comparison of time to first powerplateau for various starting powers broken down by full bitemeasurements (tissue along full length of electrode) and partial bitemeasurements (tissue along partial length of electrode).

FIG. 7 is a chart illustrating a comparison of time to first powerplateau for various maximum time values at a 20 W starting power.

FIG. 8 includes two charts illustrating a situation where transmuralityplateau is detected without a power increase.

FIG. 9 is a chart illustrating a comparison of average dZ/dt valuechanges after a power decrease, with each rise (small, medium, and largerise) detected at t=0 seconds, and a dotted line indicating the boundarybetween no rise and small rise.

FIG. 10 is a chart illustrating a comparison of the direction ofpositive acceleration in different dZ/dt values, with dotted linesindicating boundaries between rise types.

FIG. 11 is a chart illustrating a frequency plot of dZ/dt values at 1.4seconds after a power decrement.

FIG. 12 is a chart illustrating a dZ/dt calculation.

FIG. 13 is a chart illustrating an impedance profile in which a largerrise followed by a smaller rise is blanked (a), while a smaller risefollowed by a larger rise is not blanked (b).

FIG. 14 is a chart comparing lesion transmurality after ablating overfat for a previous algorithm versus an algorithm according to oneembodiment of the invention.

FIG. 15 includes two graphs illustrating the impedance profiles for aone rise step down algorithm according to one embodiment of theinvention when rise is detected and either (a) power is decreased by 5 Wor (b) power is decreased to zero.

FIG. 16 is a chart illustrating a comparison of Area Under the Curve(AUC) and energy until transmurality plateau detection between differentgroups of starting power/t_(max).

DETAILED DESCRIPTION

Before any embodiments of the invention are explained in detail, it isto be understood that the invention is not limited in its application tothe details of construction and the arrangement of components set forthin the following description or illustrated in the following drawings.The invention is capable of other embodiments and of being practiced orof being carried out in various ways. Also, it is to be understood thatthe phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having” and variations thereof herein ismeant to encompass the items listed thereafter and equivalents thereofas well as additional items. Unless specified or limited otherwise, theterms “mounted,” “connected,” “supported,” and “coupled” and variationsthereof are used broadly and encompass both direct and indirectmountings, connections, supports, and couplings. Further, “connected”and “coupled” are not restricted to physical or mechanical connectionsor couplings.

FIGS. 1 and 2 illustrate a method 100 of assessing transmurality oftissue being ablated and terminating delivery of ablation energy to anelectrode in response to a plateau in impedance of the tissue inconjunction with a detected rise in impedance. The method 100 can beimplemented during operation of an electrosurgical device to control theamount of ablation energy delivered by the device to the tissue and alsoto automatically terminate the delivery of ablation energy under certainconditions. The method 100 can be carried out by a controller having anelectrical circuit or a software program, such as, for example, amicroprocessor. The controller can be integrated into an electrosurgicaldevice or electrically connected to the electrosurgical device. Datasuch as impedance measurements and temperature measurements that areused in the method 100 can be provided by sensors carried on theelectrosurgical device. Likewise, the controller can be operably coupledto the output of the electrosurgical device to control the delivery ofablation energy to the electrode.

In general, the method 100 can monitor the tissue impedance profile orthe impedance of the tissue being ablated as a function of time. Duringthe early part of the ablation, the method 100 can gradually raise thepower level of the ablation energy being delivered, while trying todetect a flattening of the tissue impedance profile. When a relativelyflat impedance profile (or “power plateau”) is discovered, the ablationpower can be raised to a next level, as shown in FIG. 3. If there are nofurther changes in the tissue impedance profile (e.g., the impedanceprofile remains relatively flat after raising power in response to apower plateau), a transmurality plateau (TP) may be declared to exist.Transmurality, or the determination that the ablation procedure iscomplete (e.g., that the ablation extends through the thickness of thetissue to be ablated), may be indicated by any of several situationsoccurring, according to some embodiments of the invention. For example,if the total time of the ablation exceeds a minimum time delay (T_(min))following a TP declaration, transmurality can be indicated. As anotherexample, if the tissue impedance profile reaches an impedance limit(Z_(limit)) during ablation, transmurality can be indicated. As yetanother example, if a rise in a certain parameter is detected (such as arise in impedance or temperature), even if a TP has not been declared,and the rise occurs after a minimum total energy (E_(min)) has beendelivered, transmurality can be indicated. Thus, embodiments of theinvention provide a method of delivering an amount of energy toefficiently achieve a transmural ablation (e.g., reducing the timeand/or energy expended to achieve a transmural ablation), while alsominimizing the potential for over-ablation or tissue damage.

In order to prevent rapid impedance rises which can cause a highimpedance shut off (HISO), for example, the method 100 can include anegative closed loop feedback system that can be kept active throughoutthe ablation. The negative closed loop feedback system can activelylower power output of the electrosurgical device if a rise in impedanceis detected, according to some embodiments of the invention. Theresponse of the closed loop feedback system may be based on how the risein impedance is categorized, for example, according to one of threedefined rise types. Thus, power can be actively modulatedbi-directionally (e.g., positively or negatively) based on the slope ofthe impedance profile, for example, according to various embodiments ofthe invention.

FIG. 4( a) illustrates an impedance profile and power level plot versustime of an algorithm for determining transmurality, in which power mayonly be increased (and not decreased) during the delivery of ablationenergy. FIG. 4( b) illustrates a power level plot and resultingimpedance profile of a method in accordance with some embodiments of theinvention (such as method 100), in which power can be either increasedor decreased as determined by the method.

As shown in FIG. 1, the ablation is initialized (at 102). A controllerinitiates delivery of ablation energy from an electrosurgical device tothe tissue to be ablated and enters into a primary algorithm 103. Theablation energy may be delivered at a starting power, P₀. The startingpower can contribute to an early impedance rise, which can occur withinabout ten seconds following the beginning of ablation. The startingpower can also influence the rate of impedance decay until the impedancecurve flattens, thus affecting overall ablation time. In someembodiments, the starting power, P₀, can be set to about 15 W, 20 W, 25W, 30 W, or 35 W, as illustrated in FIGS. 5 and 6, or may be set toother values as deemed appropriate.

With continued reference to FIG. 1, the controller can determine (at104) if a start-up blanking period has expired. The start-up blankingperiod, which can generally cover an initial ablation period, can beprovided so that a sufficient number of measurements can be gatheredbefore attempting to determine transmurality of an ablation lesion intissue. The start-up blanking period can prevent erroneous data fromprevious ablations from being used in the analysis of the impedanceprofile and subsequent comparison to the criteria of plateaus (e.g.,power plateaus and transmurality plateaus) and rise (e.g., impedancerise and temperature rise). The start-up blanking period can be set tostart at t=0 during ablation (e.g., at the commencement of delivery ofablation energy to the tissue), and can have different lengths (e.g., itmay be programmable and/or adjustable). In one possible embodiment, thestart-up blanking period may be calculated using a formula that seeks toensure sufficient data have been acquired prior to assessingtransmurality. For example, the start-up blanking period may be definedas 1400+200*(y−1), wherein the start-up blanking period is a time periodmeasured in milliseconds, and “y” is the number of dZ/dt calculationsdesired for making transmurality assessments. If the start-up blankingperiod has not yet expired, the controller can continue to deliverablation energy at the starting power, for example. Once the start-upblanking period has expired, the controller can enter (at 106) aprocessing state in which a plurality of factors are processed ormonitored in parallel with one another, while continuing to deliverablation energy.

Various embodiments of the invention may allow the power level of theablation energy to be varied (e.g., increased or decreased). In someembodiments, a maximum time may be specified for delivery of ablationenergy at each of a number of different power levels. Thus, withreference to FIG. 1, a first factor can determine (at 108) whether amaximum time, t_(max), at the current power level has been exceeded. Thevalue for t_(max) can be the same for each power level, or it can differdepending upon the power level. Tables 1(a)-(c) below illustrateexamples of t_(max) values for each of several exemplary power levelsaccording to embodiments of the invention. It should be noted that theparticular values in Tables 1(a)-(c) are purely illustrative in nature;one of ordinary skill would be able to modify these values to achievesimilar results without departing from the scope of the invention asclaimed. FIG. 7 illustrates the impact of t_(max) on the amount of timeit can take to reach a first “power plateau.” If t_(max) has not beenexceeded, no action is taken relative to the first factor. Thecontroller can continue to deliver ablation energy and can remain (at106) in the parallel processing state. If t_(max) has been exceeded, thecontroller can exit (at 110) the primary algorithm 103 and can enter apower modulation algorithm 111, as shown in FIG. 2. This can ensure thatthe ablation energy is applied at a given power level for no longer thanthe maximum time (t_(max)) associated with that power level before thecontroller goes to the power modulation algorithm 111.

TABLE 1(a) Power Levels and Times Power plateau blanking period Max time(t_(max)), Level Power (W) (t_(min)), seconds seconds P⁻¹ 15 1.80 2.00P₀ 20 1.80 6.00 P₁ 25 1.80 6.00 P₂ 30 1.80 6.00 P₃ 35 1.80 6.00 P₄ 401.80 35.00

TABLE 1(b) Power Levels and Times Power plateau blanking period Max time(t_(max)), Level Power (W) (t_(min)), seconds seconds P⁻¹ 20 2.00 4.0 P₀25 2.00 8.0 P₁ 30 2.00 8.0 P₂ 35 2.00 8.0 P₃ 40 2.00 8.0 P₄ 45 2.0040.00

TABLE 1(c) Power Levels and Times Power plateau blanking period Max time(t_(max)), Level Power (W) (t_(min)), seconds seconds P⁻¹ 25 3.1 1.0 P₀30 3.1 5.00 P₁ 35 3.1 5.00 P₂ 40 3.1 5.00 P₃ 45 3.1 5.00 P₄ 50 3.1 30.00

As used above, a “power plateau,” or flattened impedance profile, mayoccur during delivery of ablation energy to tissue. For example, themonitored impedance of the tissue being ablated is typically observed todecrease during delivery of ablation energy to tissue. At some point,the rate of decrease of the monitored impedance begins to level off (orflatten) during energy delivery. Such a power plateau may be indicatedby a reduction in the absolute value of the slope of the monitoredimpedance during delivery of ablation energy, or by some comparablemeans of identifying a flattening of the impedance profile.

A second factor can determine (at 112) whether a power plateau, or aflattened impedance profile, has been detected. If no power plateau hasbeen detected, no action is taken relative to the second factor. Thecontroller can continue to deliver ablation energy and can remain (at106) in the parallel processing state. If a power plateau has beendetected, the controller can determine (at 114) whether there has been aprevious power decrease. If there has been a previous power decrease,the controller can go (at 110) to the power modulation algorithm 111. Ifthere has not been a previous power decrease, the controller candetermine (at 116) whether a power plateau blanking period or minimumtime per power (T_(min)) has expired. The power plateau blanking periodprevents excessive power increases from occurring within a short periodof time. It also allows time so that tissue responds to a power changebefore another power change is applied. Unlike previous algorithms whichdid not provide a decrease power, an algorithm according to the method100 allows power to decrease. This power decrement may lead to apossibility of TP detection without power ever having increased (asshown in FIG. 8). In order to prevent this problem, the power plateaublanking period, T_(min), may not apply after a power decrement,according to some embodiments. This can also reduce the possibility ofdetecting a TP at a power level less than the starting power (i.e.,P⁻¹). During the power plateau blanking period, the power is notincreased, in some embodiments. Data collected during the power plateaublanking period can be used for decision making following expiration ofthe power plateau blanking period. In some embodiments, the powerplateau blanking period, or T_(min), can be about 1.8 seconds for eachpower level, according to the example shown above in Table 1.

If the power plateau blanking period (i.e., the minimum time at thecurrent power level) has expired, the controller can go (at 110) to thepower modulation algorithm 111 (FIG. 2). If the power plateau blankingperiod has not expired, no action is taken relative to the secondfactor. Again, this means that the controller can continue to deliverablation energy and can return (at 106) to the parallel processingstate.

A third factor can determine (at 118) whether a rise has been detected.This can be an impedance rise or a temperature rise, for example. If norise has been detected, no action is taken relative to the third factor.The controller can continue to deliver ablation energy and can remain(at 106) in the parallel processing state. If a rise has been detected,the controller can determine (at 120) whether a rise blanking period hasexpired. The rise blanking period can prevent excessive power increasesdue to the detection of multiple rises within a short period of time.The rise blanking period can also provide a minimum time so thatimpedance stabilizes after a power level change or modulation (as shownin FIG. 9). In some embodiments, the rise blanking period can be about1.4 seconds. Data collected during the rise blanking period can be usedfor decision making following expiration of the rise blanking period.

If the rise blanking period has not yet expired, no action is takenrelative to the third factor. The controller can continue to deliverablation energy and can return (at 106) to the parallel processingstate. If the rise blanking period has expired, the controller candetermine (at 122) whether the total energy delivered by theelectrosurgical device is greater than a minimum energy (E_(min)).Energy (Joules) is calculated as power (Watts)Xtime (seconds). Forexample, total energy can be calculated every 0.2 seconds as follows:

${{TotalEnergy}\left( {t = {N\mspace{14mu} \sec}} \right)} = {\sum\limits_{n - 1}^{5N}\; \frac{{Power}(n)}{5}}$

If the total energy delivered by the electrosurgical device is notgreater than E_(min), the controller can go (at 110) to the powermodulation algorithm. If the total energy is greater than E_(min),transmurality can be indicated (at 124) and the ablation (e.g., thedelivery of ablation energy to a given tissue site) can be ended (at126). E_(min) can be selected to ensure that transmural lesions occur inevery operating condition according to the algorithm of method 100.E_(min) can be from about 300 J to about 500 J, in some embodiments.

A fourth factor can determine (at 128) whether the tissue impedanceprofile is greater than a maximum impedance (Z_(limit)). If the tissueimpedance profile is less than or equal to Z_(limit), no action is takenrelative to the fourth factor. The controller can continue to deliverablation energy and can remain (at 106) in the parallel processingstate. If the tissue impedance profile is greater than Z_(limit),transmurality can be indicated (at 124) and ablation can be ended (at126).

A fifth factor can determine (at 130) whether the tissue impedanceprofile is greater than a high impedance shut off (HISO) limit. If thetissue impedance profile is not greater than the HISO limit, no actionis taken relative to the fifth factor. This means that the controllercan continue to deliver ablation energy and can remain (at 106) in theparallel processing state. If the tissue impedance profile is greaterthan the HISO limit, ablation can be ended (at 126) and an error reportcan be generated (at 132). The HISO limit can correspond to a safetylimit of the tissue impedance profile. The fifth factor can limit thedelivery of ablation energy regardless of the length of time ablationenergy has been delivered, the power level of the ablation energy beingdelivered, or the indication of transmurality, etc.

As described above, a variety of conditions can cause the controller toexit the primary algorithm 103 and enter (at 110 in FIG. 1) the powermodulation algorithm 111. FIG. 2 illustrates the power modulationalgorithm 111. The power modulation algorithm 111 can be used todetermine if and how to modulate the power level of the ablation energy,such as by increasing, decreasing, or maintaining the current powerlevel.

As shown in FIG. 2, the controller can determine (at 150) if time is atzero, which corresponds to setting the starting power level. If time isnot at zero, the starting power is set (at 152) and the controller canexit the power modulation algorithm 111 and return (at 154) to theprimary algorithm 103. If the time is not zero, the controller canattempt to determine whether an impedance rise has been detected. A risecan be defined as a positive slope in the impedance profile, forexample. In some embodiments, the impedance rise can be categorizedaccording to the type of rise. For example, the magnitude of the slopemay determine which type of impedance rise is occurring, includingsmall, medium, and large impedance rises, as illustrated in FIG. 10. Todetermine the rise type, for example, a certain fraction of measuredvalues (e.g., x out of y) must have a slope magnitude, dZ/dt, thatexceeds a predetermined value, c. Tables 2(a) and 2(b) below provideexemplary values for x, y, and c for categorizing a rise in impedance asbeing a small, medium or large rise type. For example, using thecriteria provided in Table 2(a), an impedance rise would be categorizedas a “large” rise if 2 out of 4 measured values of impedance slope,dZ/dt, have a magnitude greater than 5.5.

TABLE 2(a) Plateau Variables${{{For}\mspace{14mu} n} = {1\mspace{14mu} {to}\mspace{14mu} y}};{{\frac{dZ}{dt}}_{n} > c}$Power Transmurality Rise Variable plateau plateau Rise small medium Riselarge x 4   9  3   2 2   y 5   10   5   4 4   c 1.3  1.3 1.3 3 5.5

TABLE 2(b) Plateau Variables Power Transmurality Rise Variable plateauplateau Rise small medium Rise large x 6 13 4 3 3 y 7 14 7 6 6 c 1.5 1.51.5 3.1 6.8

In some embodiments, it may be desirable to further define a small riseas being, for example, 3 out of 5 measured values of impedance slope,dZ/dt, having a magnitude between 1.3 and 3 (e.g., the slope criteriafor a medium rise). Similarly, it may be desirable in some embodimentsto define a medium rise as being, for example, 2 out of 4 measuredvalues of impedance slope, dZ/dt, having a magnitude between 3 and 5.5(e.g., the slope criteria for a large rise).

If a small rise is detected (at 156), the controller can determine (at158) if the current power is greater than a preset maximum correspondingto the small rise. In some embodiments, the preset small rise maximumpower can be 25 W, for example. If the current power level is greaterthan or equal to the preset small rise maximum, power can be decreased(at 160) by a specified amount, for example, by 5 W. At that point, thecontroller may exit (at 154) the power modulation algorithm 111 andreturn to the primary algorithm 103. If the current power is less thanthe preset small rise maximum, the controller can exit (at 154) thepower modulation algorithm 111 and can return to the primary algorithm103 without modulating the power.

If a small rise is not detected (at 156), but a medium rise is detected(at 162), the controller can determine (at 164) if the current power isgreater than a preset maximum corresponding to the medium rise. In oneembodiment, the preset medium rise maximum power level is 30 W. If thecurrent power is greater than or equal to the preset medium risemaximum, power can be decreased (at 166) by a certain amount, forexample 10 W, and the controller can return (at 154) to the primaryalgorithm 103. If the current power is less than the preset medium risemaximum, the controller can return to the small rise determination (at158) and can continue from there.

If a medium rise is not detected (at 162), but a large rise is detected(at 168), the controller can determine (at 170) if the current powerlevel is greater than a preset minimum corresponding to the large rise.In one embodiment, the preset large rise minimum can be 15 W. If thecurrent power is less than the preset large rise minimum, the controllercan return (at 154) to the primary algorithm 103 without modulating thepower. If the current power is greater than or equal to the preset largerise minimum, the controller can determine (at 172) if the current poweris greater than or equal to a preset large rise maximum. If the currentpower is greater than or equal to the preset large rise maximum (e.g.,35 W), power can be decreased (at 174) by a certain amount, for example15 W, and the controller can return (at 154) to the primary algorithm103. If the current power is less than the preset large rise maximum,the controller can increase power (at 176) by a certain amount, forexample 5 W, and can return (at 154) to the primary algorithm 103. Inone embodiment, the preset large rise maximum can be 35 W.

FIG. 11 illustrates a frequency plot of impedance slope (dZ/dt) valuesat 1.4 seconds after a power decrement. In FIG. 11, the dZ/dt valueswere categorized according to rise type (no rise, small, medium, andlarge rise), and the frequency of occurrence of each rise type is shownfor each of the following power decrement amounts: 5 W, 10 W, 15 W, and20 W. Table 3 below shows the decrement and minimum power level for eachof the three rise types.

TABLE 3 Power decrement values and minimum power Rise types Small MediumLarge Decrease amount 5 watts 10 watts 15 watts Minimum power level P₀P₀ P⁻¹

If none of a small, medium, or large rise is detected, the controllercan determine (at 178) whether the power modulation algorithm 111 wasentered due to a power plateau. If so, the controller can determine (at180) if the current power is at a maximum power or the transmuralityceiling. In some embodiments, the maximum power can be a predeterminedvalue, for example 40 W. If the current power level is either at themaximum power or at the transmurality ceiling, the controller can exitthe power modulation algorithm 111 and can return (at 154) to theprimary algorithm 103 without modulating the power. If the current poweris not equal to either the maximum power or the transmurality ceiling,power can be increased (at 182) by a certain amount, for example 5 W, insome embodiments. The controller can then return (at 154) to the primaryalgorithm 103. If none of a small, medium, or large rise has beendetected and the power modulation algorithm 111 was not entered due to apower plateau, the controller can determine (at 184) if the powermodulation algorithm 111 was entered because the maximum time (t_(max))at a power level had been exceeded. If so, the controller can determine(at 180) if the current power is at a maximum power or the transmuralityceiling, substantially as described above. If not, the controller canreturn (at 154) to the primary algorithm 103.

Returning to FIG. 1, upon return (at 154) from the power modulationalgorithm 111 to the primary algorithm 103, the controller returns (at106) to the parallel processing state. In addition, concurrently withthe controller entering the power modulation algorithm 111, thecontroller may also continue (at 186) with the primary algorithm 103.

The controller can determine (at 186) whether a transmurality plateauhas been detected. If no transmurality plateau has been detected, thecontroller can return (at 106) to the parallel processing state. If atransmurality plateau has been detected, the controller may nextdetermine (at 188) whether a minimum time per ablation (T_(min)) hasexpired. If T_(min) has not expired, no further action is taken, thecontroller can continue to deliver ablation energy and can return (at106) to the parallel processing state. If T_(min) has expired,transmurality can be indicated (at 124) and ablation can be ended (at126). The minimum time per ablation, T_(min), can be about 10-30 secondsin some embodiments, as shown below in the exemplary groups of settings(Settings A-C) of Table 4, which also provides examples of othervariable settings.

TABLE 4 Transmurality Mode Settings Variable Description Setting ASetting B Setting C t₁ Start time to check 0.2 sec. 0.3 sec. 0.1 sec.for Z_(max) t₂ Stop time to check 2.0 sec. 1.5 sec. 2.2 sec. for Z_(max)T_(max) Maximum time per  40 sec.  45 sec.  50 sec. ablation T_(min)Minimum time per  19 sec.  22 sec.  24 sec. ablation P_(max) MaximumPower  40 watts  45 watts  50 watts (watts) Scale Offset scalemultiplier 2.8 3.0 2.5 for Z_(min) Transmurality Shutoff: Manual ManualManual Shutoff Shutoff Shutoff

In some embodiments, detection of a transmurality plateau can requirethat dZ/dt be greater than or equal to 1.3 for 9 out of 10 points in adetection window, as indicated in Table 2 (above). Requiring asignificant number of the points in the detection window to satisfy thetransmurality plateau criteria may ensure that there is only one powerincrement arising from the power modulation algorithm 111 in atransmurality plateau detection window.

To reduce the possibility of severe over-ablation, a maximum totalablation time (T_(max)) for creating an ablation lesion or forperforming an ablation procedure can be imposed. In one embodiment, themaximum total ablation time is about 40 seconds, as indicated in theexample shown in Table 4. When t_(max) is reached, power delivery can beterminated regardless of the transmurality determination. In oneembodiment, the controller may indicate an error condition to the userwhen power delivery is terminated due to maximum total ablation time,t_(max), being reached. This indication can be an audible indicator, avisual indicator, or a combination of both.

In some embodiments, the tissue impedance (Z) can be measured orcalculated about every 0.2 seconds, for example. However, in somecircumstances, there can be a significant amount of noise in the signal.To reduce the effects of this noise, the data can be filtered. Oneexample of a filtering method to reduce the effects of noise can beaccomplished using a 5-point moving average of the measured impedancevalues:

$Z = \left\lbrack \frac{Z_{t - 2} + Z_{t - 1} + Z_{t} + Z_{t + 1} + Z_{t + 2}}{5} \right\rbrack$

The 5-point moving average may result in the filtered impedance laggingthe measured impedance values by about 400 msec in embodiments in whichimpedance is measured or calculated every 200 msec, for example.

The tissue impedance profile, or the rate of change in impedance perunit time (dZ/dt), can be calculated from the measured impedance values(e.g., without filtering), or from filtered impedance data (e.g., usingthe 5-point moving average), with a 3-point central differencealgorithm, as shown in FIG. 12, and as described by the followingequation:

$\frac{Z}{t}\frac{1}{2\Delta \; t}\left( {Z_{t + 1} - Z_{t - 1}} \right)$

The rate of change in impedance per unit time, dZ/dt, can therefore lagthe filtered impedance by about 200 msec in some embodiments. Toidentify regions of the tissue impedance profile as a “rise” or a“plateau,” a rolling window of dZ/dt points can be examined.

The method of assessing transmurality and terminating delivery ofablation energy to an electrode as described in relation to FIGS. 1 and2 is based on the concept of finding a flat impedance profile orplateau. When the algorithm finds a flat impedance curve, it may raisepower to a next level. If there are no further changes in the impedanceprofile, a transmurality plateau can be declared.

A plateau can be defined as a flattening of the impedance curve. Todetermine a plateau, the absolute value of a certain number (e.g., x outof y) of the dZ/dt points must be less than or equal to some definedslope value, c, wherein y is the number of points in the detectionwindow:

${{{For}\mspace{14mu} n} = {1\mspace{14mu} {to}\mspace{14mu} y}};{{\frac{Z}{t}}_{n} \leq c}$

There are two types of plateaus—power plateaus and a transmuralityplateaus—that may be defined, for example, by using different values forx and y, and having different responses (e.g., x out of y impedanceslope values meeting certain criteria). Table 2 above shows examples ofdifferent criteria for identifying power and transmurality plateaus.When a power plateau is reached, the controller can increment the powerlevel of the ablation energy to a next (e.g., higher) level. In someembodiments, a power plateau blanking period may also be established andused, whereby the criteria for identifying a power plateau is notevaluated until the completion of the power plateau blanking period.Such a power plateau blanking period may be employed, for example,following a change in power level of the ablation energy. When atransmurality plateau is reached, a transmurality flag can be set, and,in some embodiments, power cannot be increased beyond the power level atwhich the transmurality plateau was detected (e.g., the power level atwhich a transmurality plateau is identified may define a “ceiling” onthe power level, or a “transmurality ceiling,” according to someembodiments). Power can be allowed to be decreased and increasedaccording to the power modulation criteria after a transmurality plateauis identified, but the ceiling cannot be exceeded, in some embodiments,as indicated at 180 in FIG. 2. When a transmurality plateau is detectedat the same time that a power plateau is detected, the transmuralityplateau rule may supersede the power plateau rule and power may remainat the same level, according to some embodiments of the invention.

The rise blanking period may be applied in certain situations. Forexample, in some embodiments, the rise blanking period is applied onlyin situations where a given rise (e.g., impedance rise or temperaturerise) is of the same level (e.g., small, medium, or large) or below thelevel of a preceding rise. For example, the rise blanking period mayapply between successive rises of equal type, or may apply when a mediumrise occurs following a large rise, or when a small rise follows eithera medium or a large rise.

To further illustrate by way of example, if a medium rise is detectedwhile ablating at 35 W, a 10 W reduction in power may be implementedaccording to the power modulation algorithm illustrated in FIG. 2. Forthe duration of the rise blanking period, or the next 1.4 seconds, whileablating at the reduced level of 25 W, the algorithm can be blanked fromacting again on small or medium rises. This can allow time for thetissue impedance to stabilize and can reduce the rate of rise withoutoverreacting and lowering power levels excessively. However, if a largeimpedance rise should be detected during the rise blanking period, thealgorithm can immediately reduce power by 15 W or to a minimum power of15 W, for example, regardless of the blanking period (e.g., the blankingperiod is ignored). FIG. 13 illustrates the application of the riseblanking period. In FIG. 13( a), a larger rise followed by a smallerrise is blanked, while in FIG. 13( b), a smaller rise followed by alarger rise is not blanked. If impedance is still rising after the riseblanking period expires, another power reduction can be implementedaccording to the algorithm of method 100.

Table 5 below provides a number of groups of exemplary settings andcriteria (Settings A-C) that may be employed by a transmurality controlalgorithm according to some embodiments of the invention.

TABLE 5 Exemplary criteria and settings for the algorithm of FIGS. 1 and2 Setting A Setting B Setting C Starting Power, P₀ 20 W 25 W 30 WAvailable Powers 15, 20, 25, 30, 35, and 20, 25, 30, 35, 40, and 25, 30,35, 40, 45, and 40 W 45 W 50 W Power plateau criteria 4/5 pts 6/7 pts6/7 pts |dZ/dt| = 1.3 |dZ/dt| = 1.5 |dZ/dt| = 1.5 t_(max) (maximum time6 sec 8 sec 5 sec per power) 2 sec for P⁻¹ = 15 W 4 sec for P⁻¹ = 20 W 1sec for P⁻¹ = 25 W t_(mim) (power plateau 1.8 sec 2.0 3.1 blankingperiod) T_(mim) (minimum time 19 sec 22 sec 24 sec per ablation)Transmurality plateau 9/10 pts 13/14 pts 13/14 pts |dZ/dt| = 1.3 |dZ/dt|= 1.5 |dZ/dt| = 1.5 Rise criteria three rise types: three rise types:three rise types: 3/5 pts dZ/dt > 1.3 4/7 pts dZ/dt > 1.5 4/7 ptsdZ/dt > 1.5 (small), 2/4 pts dZ/dt > (small), 3/6 pts dZ/dt > (small),3/6 pts dZ/dt > 3 (med), or 2/4 pts 3.1 pts (med), or 3/6 3.1 pts (med),or 3/6 dZ/dt > 5.5 (large)) pts dZ/dt > 6.8 pts dZ/dt > 6.8 (large))(large)) Step down after rise Small: by 5 W (no less Small: by 5 W (noless Small: by 5 W (no less detection than 20 W) than 20 W) than 20 W)Medium: by 10 W (no Medium: by 10 W (no Medium: by 10 W (no less than 20W) less than 20 W) less than 20 W) Large: by 15 W (no Large: by 15 W (noLarge: by 15 W (no less than 15 W) less than 15 W) less than 15 W) AfterRise detection 1.4 sec blanking 1.4 sec blanking 1.4 sec blankingbetween rises of the between rises of the between rises of the samesize; same size; same size; 1.4 sec blanking at 1.4 sec blanking at 1.4sec blanking at med rise after large med rise after large med rise afterlarge rise; rise; rise; 1.4 sec blanking at 1.4 sec blanking at 1.4 secblanking at small rise after either small rise after either small riseafter either med or large rise med or large rise med or large riseTransmurality Transmurality plateau Transmurality plateau Transmuralityplateau indication and after T_(min); Z_(limit) and after T_(min); andafter T_(min); Z_(limit) reached; or Any type Z_(limit) reached; orreached; or of rise after E_(min) = Any type of rise after Any type ofrise after 430 J E_(min) = 380 J E_(min) = 490 J Others Ceiling: If TPN/A Ceiling: If TP declared, power level declared, power level may beModulated, may be Modulated, but may not exceed but may not exceed thepower level at the the power level at the time of TP declaration time ofTP declaration

In the method 100 of FIGS. 1 and 2, the occurrence of rapid impedancerises that might lead to a Z_(limit) or to HISO condition have beenreduced so that an ablation that fails to result in a TP will continue,running past T_(min) and potentially all the way to t_(max). However,the determination of the total amount of energy is used to provide anindication of transmurality in situations when a TP is not detected,thereby reducing instances in which ablation energy continues to bedelivered until t_(max). Energy provides a linear criteria in a relationof the formation of ablation lesions. The value for E_(min) can be basedon the maximum energy value which produces non-transmural lesions, alongwith an additional margin.

As shown below in Table 6, energy values from 340 J to 500 J with 20 Jintervals at a fixed 20 W starting power can result in varyingtransmurality outcomes. In one example, reliable transmural lesionsoccurred with any value above 400J using 20 W fixed power. The value 400J is not necessarily, however, a minimum energy, since power can bereduced to P⁻¹, or the starting power decreased by one power step (i.e.,15 W). Therefore, a margin can be added so that 420 J can be a minimumenergy value.

TABLE 6 Ablation results using fixed 20 W with various timePartial-Submerged Energy (n = 72) Full-Submerged (n = 72) (J) Time (sec)Trans (%) Width (mm) Trans (%) Width (mm) 340 17 31.2 2.3 ± 0.9 50.0 2.3± 1.2 360 18 56.3 2.2 ± 0.7 46.4 2.4 ± 1.2 380 19 100 2.3 ± 0.8 75.0 2.5± 1.1 400 20 100 2.4 ± 1.0 100 2.5 ± 1.0 420 21 100 2.4 ± 0.7 100 2.4 ±1.2 440 22 100 2.5 ± 1.1 100 2.5 ± 1.1 460 23 100 2.0 ± 0.9 100 2.9 ±0.9 480 24 100 2.9 ± 0.8 100  3.0 ± 10.7 500 25 100 2.9 ± 1.1 100 3.3 ±0.8

Table 7 below illustrates transmurality outcomes for a fixed power valueof 10 W to 40 W with 5 W intervals, using different timing to result in420 J. As shown in Table 7, the powers above 20 W result in 100%transmurality.

TABLE 7 Ablation results of 420 J using various power and timePartial-Submerged Full-Submerged Power (n = 56) (n = 56) (W) Time (sec)Trans (%) Width (mm) Trans (%) Width (mm) 10 42 56.3 2.2 ± 1.0 46.4 2.1± 1.1 15 28 100 2.5 ± 0.8 75 2.4 ± 0.9 20 21 100 2.6 ± 0.7 100 2.5 ± 1.125 17 100 2.5 ± 0.7 100 2.6 ± 1.1 30 14 100 2.8 ± 0.9 100 2.8 ± 1.0 3512 100 2.4 ± 1.2 100 2.9 ± 0.9 40 10.5 100 2.8 ± 0.8 100 3.3 ± 1.0

In some embodiments, the E_(min) (minimum energy per ablation) value of450 J can be set by adding 30 J of margin to the presumed minimum energyvalue (420 J). This minimum energy can be used to set a baseline for thetransmural lesion. Therefore, the algorithm can indicate transmuralityif there is any sign of impedance profile change (any type of risedetection) after total energy exceeds E_(min).

The method 100 of FIGS. 1 and 2 performs particularly well on fattissue. Since fat tissue includes little water, it has very highelectrical resistivity. Therefore, the fat tissue can prevent RF currentflow and can lead to a rapid impedance rise. This can cause terminationof ablations due to Z_(limit) or HISO without achieving transmurality.FIG. 14 illustrates test results using the algorithm of FIGS. 1 and 2,showing a high percentage of transmural ablation lesions, even in fattissue.

The method 100 of FIGS. 1 and 2 can also reduce over-ablation. Comparedto the previous algorithms, which maintained the power level at the timeof a TP declaration, the method of FIGS. 1 and 2 may actively lowerpower when there is an impedance rise, for example. If there isexcessive sizzling or a temperature rise, especially in thin tissue, thepower level can be maintained between 15 W and 20 W, thus lowering thetissue temperature. Table 8 below shows that the rate at which sizzlingoccurs using the method 100 of FIGS. 1 and 2 to be lower than what wouldbe expected using prior art techniques.

TABLE 8 In vitro testing results using method 100 of FIGS. 1 and 2Lesion Trans- End at Energy HISO width Sizzle murality T_(min) Time(sec) (Joules) (%) Z_(limit) (%) (mm) (%) (%) (%) Bovine 19.0 ± 0.4 483± 53 0 1.7 2.6 ± 0.5 3.3 99.0 95.8 (n = 120) Swine 19.2 ± 1.4 447 ± 47 07.4 2.6 ± 0.5 4.4 99.0 85.3 (n = 204)

The power modulation algorithm 111 shown in FIG. 2 provides for theidentification of three different rise types. However, this number canbe increased or decreased. For example, the power modulation algorithm111 can provide for the identification of one rise type. When a rise isdetected, power can be lowered either by a pre-set amount, such as, forexample, 5 W, or to the starting power, as shown in FIG. 15, andablation can continue. The power modulation algorithm 111 can providefor the identification of two rise types, small and large. When a smallrise is detected, power can be decreased by a pre-set amount, such as,for example, 5 W. When a large rise is detected, power can be decreasedby a pre-set amount, such as, for example, 15 W, or to the startingpower. Depending upon the number of rise types identified, variousfactors, including the maximum time (t_(max)) and the minimum time(T_(min)), can be adjusted accordingly.

In FIG. 16, ablation performance until TP detection was compared using astarting power of 20 W, tmax of 6 seconds, and PP criteria (4 out of 5dZ/dt values >1.3). The area under the impedance curve (AUC) and energyvalues were compared. AUC can imply impedance decaying characteristics(slope and time), while energy can imply extent of power step up. The 20W/6 second group shows smaller AUC but greater energy values compared tothe 20 W/8 second group. This can indicate relatively faster impedancedecaying while applying more power steps compared to 20 W/8 secondgroup, which matches the 25 W/8 second group.

It will be appreciated by those skilled in the art that while theinvention has been described above in connection with particularembodiments and examples, the invention is not necessarily so limited,and that numerous other embodiments, examples, uses, modifications anddepartures from the embodiments, examples and uses are intended to beencompassed by the claims attached hereto. The entire disclosure of eachpatent and publication cited herein is incorporated by reference in itsentirety, as if each such patent or publication were individuallyincorporated by reference herein.

1. A method of applying ablation energy to achieve transmurality at atissue site, the method comprising: applying ablation energy at a firstpower to the tissue site; monitoring impedance of the tissue site;identifying a rising impedance; determining a rate of increase of theidentified rising impedance; assigning a level to the determined rate;selecting a power reduction value from a group of at least two differentpredetermined power reduction values based upon the assigned level; andreducing the ablation energy applied to the tissue site by the selectedpower reduction value to a second power.
 2. The method of claim 1further comprising applying ablation energy at an initial power levelfor a start-up blanking period.
 3. The method of claim 2 wherein thestart-up blanking period is a programmable setting.
 4. The method ofclaim 1, further comprising increasing the ablation energy applied tothe tissue site to a third power in response to a power plateau.
 5. Themethod of claim 4, further comprising increasing the ablation energyapplied to the tissue site to a third power only if a minimum time atthe second power has been met.
 6. The method of claim 1, furthercomprising reducing the ablation energy applied to the tissue site to asecond power in response to a rise in impedance only if a rise blankingperiod has been met.
 7. The method of claim 1, further comprisingmonitoring total ablation energy and terminating delivery of ablationenergy if the total ablation energy exceeds a preset minimum following arise in impedance.
 8. The method of claim 1, wherein the rate ofincrease is assigned to one of at least three levels.
 9. The method ofclaim 1, further comprising indicating transmurality in response to atransmurality plateau following a rise in impedance.
 10. The method ofclaim 9, further comprising terminating delivery of ablation energy tothe tissue site in response to a transmurality plateau following a risein impedance.
 11. The method of claim 9, wherein transmurality isindicated only after ablation energy has been delivered to the tissuesite for a minimum period, t_(min), following a transmurality plateau.12. The method of claim 1, further comprising indicating transmuralityin response to the monitored impedance of the tissue site exceeding animpedance limit, Z_(limit).
 13. The method of claim 1, furthercomprising indicating transmurality in response to detecting a rise inthe monitored impedance of the tissue site occurring after a minimumtotal energy, E_(min), has been delivered.
 14. The method of claim 1,further comprising indicating transmurality in response to detecting arise in a temperature of the tissue site occurring after a minimum totalenergy, E_(min), has been delivered.
 15. A system for assessingtransmurality of an ablation procedure being performed at a tissue site,the system comprising: a sensor that determines impedance of the tissuesite; a controller adapted to communicate with the sensor to monitor theimpedance of the tissue site; the controller being adapted to: identifya rising impedance, determine a rate of increase of the identifiedrising impedance, assign a level to the determined rate, select a powerreduction value from a group of at least two different predeterminedpower reduction values based upon the assigned level, and reduce theablation energy applied to the tissue site by the selected powerreduction value.
 16. The system of claim 15, wherein the controller is amicroprocessor.
 17. The system of claim 15, wherein the system isimplemented in an electrosurgical device.
 18. The system of claim 15,wherein the controller indicates transmurality in response to a totalablation energy provided exceeding a preset minimum following a rise inimpedance.